An insulator typically comprises an insulating core that extends between two electrodes which, in operation, are maintained at significantly different electrical potentials, one of which may be earth. The insulating core may comprise a tube or a rod, which may be made of a ceramic material or of glass fiber reinforced plastics material, for example. Typically in an electrical distribution system, one end of the insulator is maintained at earth potential, and the other end is at the potential of the system, which may be 10 kV or above, for example the 375 kV electricity distribution system of the UK. At high voltages, the insulator serves to isolate the system from earth, and the higher the operating voltage of the system, the longer the insulator has to be in order to maintain the isolation. The electrical stress between the insulator electrodes results in leakage current flowing over the surface of the insulating material from high voltage to ground, and thus leads to a constant loss of power from the operating system.